This invention relates generally to paper making machines and, more particularly, to methods and apparatus in single-wire drying groups in the drying sections of paper making machines.
Most multi-cylinder drying sections of paper machines comprise two rows of drying cylinders, one above the other, with the paper web traveling in a zig-zag path between them. Air-permeable drying wires are generally used in modern installations.
The web is carried through the drying section in a single-wire and/or twin-wire conduction. In single-wire conduction drying sections, the same drying wire supports the web as it passes from one drying cylinder to another as well as on the run between the cylinder rows. In twin-wire conduction drying sections, separate upper and lower wires are used and the web has a free, unsupported run as it travels from one cylinder row to the other.
One of the drawbacks of conventional multi-cylinder drying sections wherein single-wire conduction is used is the tendency of the web to become detached from the surface of the drying wire on those cylinders where the web is situated on the outer surface of the drying wire. This tendency to separate from the drying wire is increased by the overpressures which are induced in the entrance nips formed between the drying wire and cylinder surfaces. Detachment of the web from the drying wire may result in web breakage or, at the least, in bagging and wrinkling of the web.
Regarding the state of the art to which this invention pertains, reference is made to DE-OS No. 31 32 040 and to Finnish patents Nos. 69143 and 70275, all of J.M. Voith GmbH, as well as to Finnish patent application Nos. 812089, 851533 and 862413. The last-mentioned patent application discloses drying groups in which the distances between the peripheries of the drying cylinders and the gaps between drying cylinders and suction-deflection rolls are minimized, and the diameters of the drying cylinders and the diameters of the suction-deflection rolls are selected such that the covering sectors of the web-carrying drying wire are within the range of between about 210.degree. and 270.degree., most preferably between about 240.degree. and 260.degree..
The trend in design of single-wire conduction drying sections are constructions wherein the steam-heated lower drying cylinders are replaced by smaller diameter lower cylinders which may be non-heated or guide rolls. This, in combination with the trend towards increasing paper machine speeds, imposes particularly high standards for arrangements by which positive contact of the web with the wire surface is maintained when the web is positioned on the outer surface of the curved deflection sector of the lower roll.
The present invention relates to drying sections including single-wire type drying groups. In particular, the invention relates to a single-wire drying group of a multi-cylinder dryer comprising a plurality of drying cylinders having axes substantially situated in a common plane and located outside the loop of the drying wire. The web-carrying drying wire is conducted over covering sectors of the drying cylinders, preferably having an extent greater than 180.degree., and deflection rolls or the like are arranged between successive drying cylinders within the loop of the drying wire, the web-carrying drying wire passing over a deflection sector of the deflection roll with the web situated on the outer surface of the drying wire.